Induction cooking devices, such as range cooktops, include a resonant power inverter circuit having an induction heating coil, commonly called a work coil, for receiving a high-frequency current, which in turn generates a high-frequency magnetic field coupled to cookware of metallic material to induce electric current therein for heating the cookware, and any contained food. In using such devices the cookware, such as a pan, is placed on a cooking surface of the range cooktop adjacent to the work coil. The high-frequency magnetic field induces eddy-currents in the metallic body of the cookware. Heat is generated in the body of the cookware as an eddy-current loss, due to the electrical resistance of the material of the cookware opposing the induced eddy-currents. Thus, it is desirable to use cookware made of magnetic metals with high electrical resistance. For this reason, the preferred cookware for induction heated cooking is generally made of materials such as iron or stainless steel.
When the cookware is not present on or is removed from the induction cooking apparatus during or after the cooking operation, the work coil loses its coupled load, referred to as a non-load state. With no load, the input impedance of the resonant circuit is enormously decreased, so that the high-frequency current in the resonant circuit greatly increases, frequently to destructive levels. This phenomenon has been used to determine the presence or absence of cookware on the induction cooking device, by sensing the high-frequency current with a current transformer. When the sensed current exceeds a predetermined value, a prescribed control circuit de-activates the power inverter circuit. As a result, the induction cooking device is protected from an erroneous and possibly destructive operation in the non-load state.
In addition, other approaches in protecting the power inverter circuit have been taken for detecting an unsuitable load on the power invention circuit in an inductive heating apparatus. For example, U.S. Pat. No. 4,356,371 discloses an inductive heating apparatus having a detection circuit which compares the input and output parameters of the inverter and latches a bistable device when the input power parameter is smaller than the output parameter, whereby the bistable device shuts down the inverter to prevent small objects from being overheated. Also, U.S. Pat. No. 4,686,340 discloses an inductive heating apparatus having a detecting circuit for detecting an input AC power and an excitation current for an inverter, so that the levels thereof are compared to discriminate whether or not a load is suitable.
Some approaches of protecting the power inverter circuit not only detect variations in the load, but also compensate for such variations. For example, U.S. Pat. No. 4,820,891 discloses a control circuit for controlling an inductive heating device in response to an impedance detection circuit and an inverter frequency detection circuit. Also, U.S. Pat. No. 4,115,676, uses a current transformer to sense the current flowing through a work coil, and provides a corresponding output as an input to control circuitry which controls the conductance of a power inverter switching transistor in accordance with the magnitude and direction of current flowing through the work coil to compensate for variations in the size, shape, and material of the cookware.
Previously developed induction cook-top devices have not provided adequate control systems for controlling the energization, deenergization and variation of heating levels in the induction heating coils of such cooking apparatus. Power inverter circuits in induction heating devices include a switching component which typically must operate at nearly maximum current capacity. In order to prevent early failure of this critical component, precise timing and real time monitoring of the power inverter circuit is required. For example, induction cooking systems are connected to various line power sources, and thus, such systems should exhibit a wide tolerance of these various power sources in which input line voltage may be low, high, temporarily low or high, noisy, or possibly all of the above.
The functions of the electronic circuitry in induction cooking systems are generally two fold. First, a power inverter circuit produces the power to perform the cooking and second, a power control, timing, and monitoring circuit operates the induction system and provides convenient control for a user.
Some induction heating systems, such as that disclosed in U.S. Pat. No. 4,429,205, rely solely on analog circuitry to provide power, control, timing and monitoring of the power inverter circuit. Still other circuits have used both analog and discrete digital components in the power inverter control circuitry, such as for example, U.S. Pat. Nos. 4,115,676; 4,356,371 and 4,617,442. Such induction systems using monitoring and control circuitry fabricated solely from hard-wired discrete components, however, cannot be easily modified to change the operating characteristics of the system, and troubleshooting such systems can be difficult.
Other systems, such as U.S. Pat. Nos. 4,308,443; 4,453,068 and 4,511,781 have relied primarily on a microprocessor to provide power control, timing and monitoring of the power inverter circuitry. Since the advent of microprocessor controls, an accepted design criteria has been to incorporate the power control, timing and monitoring circuitry into a programmable microprocessor device, which is a favorable design from a cost standpoint and for ease of product change. Using a microprocessor to provide for the timing of the power inverter circuit, however, has a significant draw back. Microprocessors are sensitive to powerline fluctuations which can cause program errors and produce random timing outputs. Random timing outputs from the control circuitry will almost inevitably cause semiconductor components of the power inverter circuit to fail. As a result, the reliability of the induction cooking system is seriously compromised. Thus, induction heating systems incorporating microprocessors for generating the critical timing signals to control a power inverter circuit are unsatisfactory for reliable operation and optimum reliability.
Therefore, an improved induction heating apparatus is needed which overcomes these deficiencies in the prior art.